Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
  • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
  • G01N27/403—Cells and electrode assemblies
  • G01N27/406—Cells and probes with solid electrolytes
  • G01N27/407—Cells and probes with solid electrolytes for investigating or analysing gases
  • G01N27/4073—Composition or fabrication of the solid electrolyte
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
  • C25B11/051—Electrodes formed of electrocatalysts on a substrate or carrier
  • C25B11/073—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material
  • C25B11/091—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material consisting of at least one catalytic element and at least one catalytic compound; consisting of two or more catalytic elements or catalytic compounds
  • C25B11/093—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material consisting of at least one catalytic element and at least one catalytic compound; consisting of two or more catalytic elements or catalytic compounds at least one noble metal or noble metal oxide and at least one non-noble metal oxide
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
  • H01M4/90—Selection of catalytic material
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/10—Fuel cells with solid electrolytes
  • H01M8/12—Fuel cells with solid electrolytes operating at high temperature, e.g. with stabilised ZrO2 electrolyte
  • H01M8/1213—Fuel cells with solid electrolytes operating at high temperature, e.g. with stabilised ZrO2 electrolyte characterised by the electrode/electrolyte combination or the supporting material
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
  • H01M4/90—Selection of catalytic material
  • H01M4/9016—Oxides, hydroxides or oxygenated metallic salts
  • H01M4/9025—Oxides specially used in fuel cell operating at high temperature, e.g. SOFC
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
  • H01M4/90—Selection of catalytic material
  • H01M4/9041—Metals or alloys
  • H01M4/905—Metals or alloys specially used in fuel cell operating at high temperature, e.g. SOFC
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
  • Y02E60/30—Hydrogen technology
  • Y02E60/50—Fuel cells

Definitions

• This invention relates to composite electrodes which are intended for use in solid electrolyte devices.
• Such devices may include, for example, oxygen sensors, oxygen pumps, fuel cells, steam electrolysis cells, and electrochemical reactors.
• Solid electrolyte devices of interest usually consist of an oxygen ion conducting electrolyte membrane held between two electrodes.
• the examples of solid electrolyte materials include zirconia, both partially or fully stabilized by the addition of ealcia, magnesia, yttria, scandia or one of a number of rare earth oxides, and thoria or ceria doped with ealcia or yttria or a suitable rare earth oxide.
• the electrodes for these devices usually consist of porous coatings of metals such as Pt, Ag, Au, Pd, Ni and Co or metal oxides with good electronic conductivity.
• the electrodes participate in the charge transfer reaction between gaseous oxygen molecules or fuels (such as hydrogen, carbon monoxide or methane) and oxygen ions in the solid electrolyte by donating or accepting electrons.
• the electrode may also help to catalyse the reaction.
• Pt the most commonly used electrode material on solid electrolyte oxygen sensors and oxygen pumps, shows high catalytic activity at temperatures above 600-700°C for the oxygen charge transfer reaction (O 2 + 4e #20 2- ) at the electrode/electrolyte interface.
• the physical and chemical characteristics of the electrode play an important role in determining the speed of response and efficiency of the solid electrolyte devices.
• the potentiometric or Nernst sensor consists of an oxygen ion conducting membrane of a solid electrolyte (with negligible electronic conductivity) such as fully or partially stabilized zirconia and two electrodes reversible to O 2 /O 2- redox equilibria. If both electrodes of such a cell are exposed to different oxygen partial pressures an emf is established across the cell which with respect to air as the reference atmosphere is given by the nernst equation:
• the current carrying capabilities of the electrodes are not important although electrodes with high charge transfer rates are required especially for low temperature (below 600°C) applications of the sensors.
• the solid electrolyte oxygen sensors with noble metal electrodes are generally used above 600-700°C. Below these temperatures they suffer from slow response rates, high impedance - susceptibility to electrical noise pick-up, large errors and exceptionally high sensitivity to impurities in the flue gases.
• Other solid state electrochemical devices such as oxygen pumps, fuel cells, steam electrolysers and electrochemical reactors may consist of a tube of fully or partially stabilized zirconia electrolyte with electrodes coated both on the outside and inside of the tube.
• a number of such cells may be connected in series and/or parallel to achieve the desired characteristics.
• an oxygen pump a number of cells, may operate in conjunction to increase rhe yield of oxygen.
• a number of the small cells may be connected in series and parallel to increase the total current and voltage output as maximum theoreticail voltage achievable from a single cell is around 1.0-1.5 volts.
• the current carrying capacity and hence the overall efficiency of the cell is determined by (i) the electrode/electrolyte interfacial resistance to charge transfer reaction and (ii) the electrolyte resistance.
• the interfacial resistance to charge transfer depends mainly on the electrochemical behaviour, and physical and chemical nature of the electrode. Because of the high electrode and electrolyte resistance at low temperature, the cells need to be operated at temperatures in the vicinity of 900-1000°C. For optimum efficiency and increased cell life it is essential that these cells be operated at lower temperatures. The volt-age losses across the electrolyte can be reduced by the use of thin and mechanically strong electrolyte films. Since conventional metal or metal oxide only electrodes have high electrode resistance at lower temperatures, it is necessary therefore to devise better electrode materials in order to reduce overpotential losses across the electrode/electrolyte interface.
• composite electrodes semiconductor metal oxides with either electron (n-type) or hole (p-type) conduction when combined- with noble metals such as Pt (referred to here-in-after as composite electrodes) are much better electrodes than either metal or metal oxide only electrodes. These electrodes when used in oxygen sensors lower their operating temperature to 300°C, well below the 600°C achievable with conventional metal or metal oxide electrodes. In addition when these composite electrodes are used in other solid electrochemical devices, increase their efficiency and enable them to be operated at temperatures lower than those achievable with conventional metal or metal oxide only electrodes. Moreover these composite electrodes have superior physical characteristics such as resistance to grain growth and better adhesion to the electrolyte surface when compared with metal or metal oxide electrodes respectively.
• the improved electrochemical behaviour is due to both constituents of the composite electrode participating in dissociation/diffusion/charge transfer reactions near and at the electrode/electrolyte interface.
• One objective of the present invention therefore is to provide electrodes for solid electrolyte oxygen sensors which enable such sensors to generate ideal (i.e. Nernstian) emfs under oxygen-excess gaseous condition at temperatures substantially below those at which conventional noble metal or metal oxide electrodes begin to show non-ideal behaviour.
• a further objective is to provide electrodes which retain their good low temperature behaviour after exposure to temperatures as high as 900°C.
• solid electrolyte devices such as oxygen pumps , fu e l cells electrochemical reactors and steam electrolysis cells, the low electrode/electrolyte interfacial resistance to charge transfer reactions and, consequently, the current carrying capabilities and current potential characteristics of the electrodes are of utmost importance as distinct from potentiometric oxygen sensors.
• a further objective of the present invention is to provide electrode materials for oxygen pumps, fuel cells, electrochemical reactors and steam electrolysers to increase their efficiency, reduce energy losses and increase their useful life time by operation at lower temperatures.
• a composite electrode material for use in solid electrolyte devices which comprises a mixture of a noble metal and a semiconducting metal oxide with either electronic (n-type) or hole (p-type) conductivity.
• the noble metal is platinum, silver, gold, iridium or rhodium or palladium or mixtures or alloys of any two or more of these metals.
• the semiconductor metal oxide may be chosen from any suitable oxide which is a good electronic conductor and possesses other required attributes such as thermal stability and chemical compatibility with the solid electrolyte. Usually the oxide will be selected from semiconducting oxides of one or more of the Transition Metals, Lanthanides or Actinides. In this specification the "Transition Metals" are those having Atomic Nos. 21 - 30, 39 - 48 and 72 - 80.
• the composite electrode material may be provided in the form of a suface layer on a solid electrolyte body.
• the surface layer may consist of a thin porous coating of a mixture of the noble metal(s) and particles of the semiconducting oxide.
• the invention also includes methods for producing the composite electrode materials of the invention.
• the components of the electrode material may be applied to a solid electrolyte surface by any suitable known coating method, such as painting, sputtering, ion implantation, spraying or other in-situ chemical or electrochemical techniques. It is preferred to prepare the semiconducting oxide material before application although it is possible (where appropriate) to apply precursors or individual components of the oxide material (or compounds which will produce them) in intimate admixture and to produce the semiconducting metal oxide by sintering the coating.
• the noble metal is applied with the oxide material (or its component oxides). Either the elemental metal or a suitable compound which can be heat-decomposed to the metal can be used.
• the electrode materials of this invention permit the construction of solid state electrochemical cells with a low interfacial impedance between the electrode and the solid electrolyte. Similar devices with conventional noble metal or metal oxide only electrodes show a much higher electrode/electrolyte interfacial impedance.
• the electrodes materials of the present invention also permit the construction of oxygen sensors whose performance under typical air-excess combustion conditions conform to the Nernst equation down to temperatures as low as 300°C.
• Sensors equipped with electrodes of the present invention may therefore be used to measure the oxygen potential of gases (e.g. boiler flue gases, internal combustion engine exhausts) in the temperature range 300°C-700°C, where sensors with. conventional electrodes require supplementary heating to generate Nernstian emfs.
• the electrodes of the present invention enable sensors to operate with supplementary heating to bring them to a temperature in the range of 300°C-400°C, whereas sensors with conventional electrodes are generally heated above 700°C.
• the lower operating temperature reduces the explosion hazard associated with maintaining heated sensors in the flue of a combustion device such as a boiler.
• the invention also includes solid electrolyte devices which incorporate a solid electrode in accordance with the invention.
• Figure 1 shows a longitudinal section of an oxygen sensor embodying the electrodes of the invention
• Figure 2 shows one form of a cell for use in solid electrochemical devices such as oxygen pumps, fuel cells, electrochemical reactors or steam electrolysers;
• Figure 3 is a graph showing plots of the electrodes resistance (Ro) versus composite electrode [consisting of (U 0.38 SC 0 .62 )O 2+x and PtO 2 ] composition at three different temperatures;
• Figure 4 is a graph showing a plot of the electrode time constant, o versus composite electrode [consisting of (U 0.38 Sc 0 .62 )O 2+x and PtO 2 ] composition at 600°C;
• Figure 5 is a graph showing a plot of the electrode resistance (Ro) versus composite electrode [consisting of CrNbO 4 and PtO 2 ] composition;
• Figure 6 is a graph showing a plot of the electrode time constant, versus composite electrode [consisting of CrNbO 4 and PtO 2 ] composition
• Figure 7 is a graph comparing current voltage characteristics of a metal oxide (CrNbO 4 ) electrode with the composite electrode constituting 75 wt% of this metal oxide + 25 wt% PtO 2 ;
• Figure 8 is a graph showing the performance of oxygen sensors provided with one composite electrode material of the present invention including comparative results for sensors provided with either individual metal oxide or metal constituting the composite electrode;
• Figure 9 - 17 are graphs comparaing the sensor performance of various semiconducting metal oxides with the composite electrodes constituting that metaL oxide and Pt (added as PtO 2 );
• Figure 18 is a graph comparing the individual performances of CrNbO 4 and PtO 2 , electrodes with composite electrodes consisting of the two materials in various ratios.
• FIG. 1 A hollow ceramic body 10 is closed at one end by a solid electrolyte disc 11. Electrodes 12 and 13 of the present invention are located at the inner and outer surfaces, respectively, of disc 11. Electrical contact to the electrodes is made by metal wires 14 and 15. Wire 14 is pressed against electrode 12 by spring loading (not shown), applied by means of insulating tube 16. Tube 16 may also be used to convey a reference gas, e.g., air, to electrode 12. if tube 16 is a multi-bore tube, it may also carry a thermocouple (wires 14 and 17) to determine the temperature of electrolyte disc 11, in which case wire 14 form one leg of the thermocouple.
• a reference gas e.g., air
• wires 14 and 15 for electrical contact to the electrodes 12 and 13 is to use coatings, e.g., of platinum, gold, palladium, silver or their alloys, or of the electrode material itself, on the inside and outside surfaces of ceramic body 10, extending from electrodes 12 and 13 to the open end of the sensor.
• coatings may completely cover surfaces of ceramic body 10, or they may consist of continuous strips covering only part of ceramic body 10.
• outer electrode 13 is not required. Electrical contact must be made to the molten metal in the vicinity of the sensor, using an electrical conductor such as wire 15 attached to the sensor but not direct contact with solid electrolyte disc 11, or using a metallic coating on the outer surface or ceramic body 10 as hitherto described, or using a separate electrical conductor adjacent to the sensor. For measurements over extended periods, it is essential that the external electrical contact not dissolve in, or otherwise be attacked by, the molten metal. If a gaseous, reference, e.g., air, is used the internal electrode 12 and electrical contact 14 are required.
• a gaseous, reference e.g., air
• FIG. 2 One form of a cell to be used in solid electrochemical devices other than oxygen sensors is shown in Figure 2.
• a hollow porous ceramic substrate body 20 is coated with a thin layer 21 of the electrodes of the present invention.
• the impervious electrolyte layer 22 is then formed on this electrode by a suitable combination of ceramic processing techniques.
• the outer electrode layer 23 of the materials of the present invention is then formed on the outer surface of the electrolyte layer.
• An alternative to this arrangement is not to use a porous substrate support but to coat electrodes of the present invention both on the inside and outside of a presintered electolyte tube.
• the materials were selected from a wide spectrum of semiconducting oxides. These included: (i) different crystal structure, (fluorite, rutile, orthorhombic, hexagonal, monoclinic etc.); (ii) varying degree of nonstoichiometry (e.g.
• the simple transition metal and rare earth oxides were obtained from chemical manufacturing companies. Solid solutions and compounds between two or more metal oxides were prepared by one of the following methods: (i) Metal salts to give required composition were dissolved inaqueous media followed by coprecipitation with aqueous ammonia solution. The coprecipitated powder was dried and calcined at high temperatures to complete the reaction.
• the second method involved dissolving the metal nitrates (in the desired quantity) in water and carefully drying the solution. This was followed by grinding the dried material and firing at high temperatures.
• transition metal oxides were mixed thoroughly in ethanol, dried and heated at high temperatures. In many cases it was necessary, to grind and refire the powder several times to complete the reaction.
• Fine pastes of various oxides l isted in Tabl e 1 in triethyl ene glycol were prepared by grinding the powdered oxide with a 25 percent solution of triethylene glycol in ethanol until the ethanol had evaporated. The procedure was repeated several times until a uni form and consi stent paste was obtained .
• Fine pastes of composite electrodes comprising a mixture in triethylene glycol of 25 weight percent platinim dioxide and 75 weight percent of the metal oxide were prepared by the procedure described in example 2.
• the compositions of the composite electrodes are given in Tabl e 4.
• the composi te electrodes described in exampl e 3 (Tabl e 2) were painted on both flat faces of sintered di scs of 7 molX Y 2 O 3 + 93 mol % ZrO 2 (YSZ7) electrolyte (diameter 9.6-9.7 mm, thickness ⁇ 2.5 mm and densi ty - 95X of theoretical for the el ectrolyte discs) and heated to 600°C for 15 hours in air.
• the electrode resistance (which determines the rate of the oxygen exchange reaction at the electrode/electrolyte interface) and the time constant, to which
• the electrode resistance, R o has been plotted against weight percent of platinum dioxide at several temperatures.
• the graphs show a minimum between 15 and 35 wt% PtO 2 .
• the initial decrease of about an order of magnitude in the electrode resistance, on addition of a mere 15 wt% PtO 2 is dramatic.
• PtO 2 content above 35 wt% the electrode resistance increased sery rapidly.
• the time constant and the electrode resistance of composite electrodes containing 25-40 wt% PtO 2 are at least an order of magnitude lower than those for either constituent of the composite.
• two cells In order to determine current voltage characteristics two cells, one provided with metal oxide only (CrNbO 4 ) and the second cell with a composite electrode consisting of 75 wt% of CrNbO 4 + 25 wt% PtO 2 were prepared, by painting the respective electrode on both flat sides of sintered discs of 10 mol% yttria + 90 mol% zirconia electrolyte. The temperature of each cell was raised to 700°C in air and the cell left to equilibrate for several horus with the surroundings before making measurements on that cell. The current voltage characteristics were recorded over several temperatures between 500° and 700°C with a galvanostatic current interruption technique.
• Oxygen sensors of the type shown in Figure 1 were prepared using the electrodes described in Tables 1 to 4.
• the electrolyte disc used comprised a sintered mixture of 50 weight percent alumina and 50 weight percent of a zirconia-scandia solid solution containing 4.7 mol% scandia.
• the sensor bodies were prepared by high temperature eutectic welding of solid electrolyte discs on to alumina tubes.
• Complete sensors were prepared by painting the electrode under test (Pt, metal oxide or a composite consisting of PtO 2 and the metal oxide) on both flat sides of the welded electrolyte disc.
• the sensor assemblies were slowly heated to 600°C to burn off the triethylene glycol and in the case of composite electrodes also to decompose PtO 2 to Pt.
• Ihese Values of resistances also include the electrolyte resistance which (as determined by impedance dispersion analysis of a number of cells) is around .6-1.2K ohm at 600°C.
• the resistance of sensors provided with composite electrodes is invariably much lower than those provided with semiconductor metal oxide only electrodes. Because of the much higher resistance of sensors provided with metal oxide only electrodes, they were extremely sensitive to electrical noise pick-up at temperatures below 400-500°C. By contrast sensors with composite electrodes showed no such sensitivity even at temperatures as low as 350°C.
• the sensors with oxide only or platinum electrodes exhibited much higher sensitivity to changes in the gas flow rate than those with the metal oxide + PtO 2 electrodes.

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• 1986
  • 1986-10-15 WO PCT/AU1986/000305 patent/WO1987002715A1/en not_active Application Discontinuation
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  • 1986-10-15 JP JP61505530A patent/JP2541530B2/ja not_active Expired - Fee Related
  • 1986-10-28 IN IN786/CAL/86A patent/IN168554B/en unknown

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